Dielectric properties of cobalt substituted M-type barium hexaferrite prepared by co-precipitation

Transcription

1 J Mater Sci: Mater Electron (2007) 18: DOI /s Dielectric properties of cobalt substituted M-type barium hexaferrite prepared by co-precipitation Philip Shepherd Æ Kajal K. Mallick Æ Roger J. Green Received: 25 May 2006 / Accepted: 16 October 2006 / Published online: 14 November 2006 Ó Springer Science+Business Media, LLC 2006 Abstract Barium hexaferrite was synthesised via the co-precipitation method using high purity nitrates, oxides and carbonates of iron (III), barium (II) and ammonium hydroxide. Once a phase pure sample of barium hexaferrite was obtained, it was doped, by weight, with 1, 2, 3, 5, 10, 15, 20 and 30% cobalt oxide (Co 3 O 4 ). The addition of cobalt to the BaM had the effect of reducing the permittivity and loss tangent until a doping of 10% whereupon it remained constant at around 9. Thermogravimetric (TG) studies of green bodies showed decarboxilation to occur at around 825 C and the transformation of residual Co 3 O 4 to Co 2 O 3 at around 900 C. The X-ray diffraction (XRD) studies confirmed the Co ions substituting in the iron sites until a doping level, of 10 15% where the structure underwent a transition to one more closely resembling the W-type hexaferrite. The measured densities were found to vary with doping levels, with a maximum of 4.45 g/cm 3 at 1% Co doping. properties suitable for RF and microwave applications. The parent ferrite, BaFe 12 O 19 is a hexagonal hard ferrite within the space group of P63/mmc [4]. Due to the high interest of this compound as a material suitable for magnetic recording, much research has been directed towards the production of suitable cation doping. The barium ferrite, with cobalt doping has been the subject of many such investigations [5 7]. The preparation techniques as well as the structural doping of Co ions influence the magneto-dielectric properties of this compound. This paper examines the influence on TGA, crystal evolution from M-type to W-type hexaferrite, complex relative permittivity and density caused by the doping of Co ions. Hexaferrites are classified into five main types depending upon chemical formula and crystal structure, as denoted by M, W, Y, Z and X-type. Magnetic property measurements of the series of compounds are still in progress and will be reported at a later date. 1 Introduction Hexagonal barium hexaferrite (BaFe 12 O 19 ), often denoted as M-type BaM, is widely used for various important electronic applications such as permanent magnets, particulate media for magnetic recording and microwave devices [1 3]. Ferrites and garnets are ferromagnetic oxides with dielectric and magnetic P. Shepherd K. K. Mallick (&) R. J. Green School of Engineering, University of Warwick, Coventry CV4 7AL, UK k.k.mallick@warwick.ac.uk 2 Experimental 2.1 Synthesis A chemically reliable co-precipitation route as well as the classic solid state synthesis method from relevant oxides was used in the preparation of a stoichiometric composition of BaM. The solution route involved a slow and drop wise addition of excess ammonium hydroxide (ph = 12) to the aqueous solution consisting stoichiometric amounts of nitrates of Ba and Fe (99.99% purity, Aldrich) that resulted in the co-precipitation of a gel like cake. The cake was thoroughly washed with

2 528 J Mater Sci: Mater Electron (2007) 18: deionised water and purified ethanol, dried at 105 C and was subsequently heated to various temperatures. The ceramic method involved firstly the formation of stoichiometric monoferrite, BaFe 2 O 4 using BaCO 3 (99.999% purity, Aldrich) and Fe 2 O 3 (99.98% purity, Aldrich) as raw constituents, followed by further reaction with excess Fe 2 O 3 to obtain barium hexaferrite. Following intermittent heating at 800 C for 3 h and grinding (in an agate mortar and pestle) various samples were formed and different percentages of Co 3 O 4 by weight were added to the phase pure BaFe 12 O 19. Following further intermittent heating at 800 C for 3 h and grinding the final material samples were pelletised using a Specac uniaxal press to MPa producing disk samples of 16 mm in diameter. Sintering of these pellets was carried out at temperatures ranging from C, the latter being the final sintering temperature. Six samples of each barium cobalt ferrite composition produced by both methods were subjected to identical temperature regimes and time intervals. 2.2 Density measurements The density of the samples from the green to a sintered state was measured using the well known liquid displacement Archimedes technique. The measurements were carried out to record the change in density following various sintering treatment protocol. The equipment used in the technique is of such accuracy as the results are accurate to ±0.01 g/cm X-ray diffraction measurements Chemical composition analysis was carried out by powder XRD. XRD patterns for samples of 0, 3, 15 and 30% Co 3 O 4 doping treated at various temperatures and times were recorded in the region of 2h = withastepscanof0.02 /min on a Philips diffractometer (Model PW1710) using CuK a radiation. Automated powder diffraction software package that included both standard ICDD and calculated ICSD diffraction files was used to match the evolving phases. Cell parameters were calculated and further refined using linear regression procedures (Philips APD 1700 software) applied to the measured peak positions of all major reflections up to 2h =90. The crystallite size was determined using the wellknown Scherrer formula, [8] from the line broadening of diffraction profile of the strongest peak. The formula, excluding the effects of the machine broadening to minimise errors, is given below: D ¼ kk cos h ð1þ h 1=2 where D = average size of the crystallites, k = Scherrer constant (k = 0.9). k = wavelength of radiation (k CuK\alpha = Å), h 1/2 = peak width at half height (FWHM measured in radians) and h corresponds to the peak position measured in radians. The measured values were corrected for the effects caused by instrumental broadening. 2.4 Scanning electron microscopy Determination of the morphology was performed using Scanning Electron Microscopy (SEM). A Philips Cambridge Stereoscan was used to determine the phase pure and Co ion doped barium hexaferrite particles as well as to observe related microstructural features of the ferrite particulates. 2.5 Dielectric measurements An Impedance/Material Analyser E4991A with a 16453A test fixture for dielectric material measurements, from Agilent Technologies was used to determine the high frequency dielectric properties. The frequency relevant was 1 MHz 1 GHz and measured at ambient temperature (25 C). The classic parallel plate method is used in the test fixture to determining the relative complex permittivity and loss tangent of the material. The parallel plate capacitor procedure involves placing the sample material in between two circular plates; the load is then measured by an LCR meter. The real part of the complex relative permittivity (e r ) is calculated from the capacitance and the imaginary part of the complex relative permittivity (e r ) is calculated from the measurement of the dissipation factor. A calibration procedure performed on the test fixture using an OPEN, SHORT and a known LOAD state before the DUT was measured as this reduces the errors in the system [9]. The technical manual for the E4991A test equipment and the 16453A test fixture show illustrates an error factor of approximately 2% across the bandwidth of 1 MHz 1 GHz. The error becomes too great below and above those frequencies due to resonances within the test fixture.

3 J Mater Sci: Mater Electron (2007) 18: Thermogravimetric and differential thermal analysis Samples of unsintered, green material of BaM containing 0, 3, 10 and 30% Co doping were analysed by TG and differential thermal analysis (DTA). A PerkinElmer Pyris Diamond TG/DTA was used, over the temperature range of C, at a rate of 20 C per minute, in static air using platinum crucibles. 3 Results and discussion 3.1 Thermogravimetric and differential thermal analysis The TG/DTA plots are shown in Fig. 1a d, for the green unsintered compound of BaCO 3, Fe 2 O 3 and Co 3 O 4 doping of 0, 3, 10 and 30% by weight. For the BaM sample the weight loss is incremental and an overall loss of about 4% is indicated. In all cases, there is an appearance of an endotherm in the DTA curve at around 800 C. The DTA peak is attributed to the decarboxilation of BaCO 3, reported to take place at 1055 C for pure carbonate, [10] and around 800 C for the mixture of carbonate and iron oxide [11]. The DTA peak at around 900 C, as shown in Fig. 2c and d, for doping levels of 10 and 30% corresponds to the transformation of residual Co 3 O 4 to Co 2 O 3. These DTA peaks are not clearly delineated in Fig. 2a and b for lower doping levels. The completion of formation of the hexaferrites is indicated at around 1011 C and 1050 C for the W-type ferrites, shown in Fig. 2d. 3.2 Variation of density Figure 2 shows the isothermal and isochronal variation of the mean average value of density for doped BaM within the sample population at various doping regimes over temperatures and time intervals shown, including max and min bars for each mean value. For the un-doped BaM, following the final sintering at 1200 C for 2 h the mean average density was found to be 4.22 g/cm 3 with a minimum of 4.17 g/cm 3 and a maximum of 4.29 g/cm 3, density then peaked at 4.45 g/cm 3 with a minimum of 4.40 g/cm 3 and a maximum of 4.49 g/cm3 with 1% doping. With increasing the dopant Fig. 1 (a) Thermogravimetry (TGA) and differential thermal analysis (DTA) of as synthesised co-precipitated BaM cake. (b) TGA and DTA of BaM cake with 3% Co doping. (c) TGA and DTA of BaM cake with 10% Co doping. (d) TGA and DTA of BaM cake with 30% Co doping by weight

4 530 J Mater Sci: Mater Electron (2007) 18: Fig. 2 Effects on density with variations in sintering temperature, time and doping concentration percentage the density dropped, whereupon at 20% Co 3 O 4 doping the density was 3.5 g/cm 3 with a minimum of 3.46 g/cm 3 and a maximum of 3.59 g/cm 3.At 30% doping the density rise sharply to 4.21 g/cm 3 with a minimum of 4.15 g/cm 3 and a maximum of 4.23 g/cm 3. During the lower temperature sintering regimes the density varied little with time, temperature and doping levels, all remaining around the 3.2 g/cm 3 level with a minimum of 2.81 g/cm 3 and a maximum of 3.9 g/cm Crystal structure and particle morphology Figure 3 shows the typical XRD patterns of the sintered BaM (0% doping of Co 3 O 4 ) together with Co 3 O 4 doped BaM at 1, 3, 10 and 30% by weight. The figures also include the standard (JCPDS no: ) [12], for barium hexaferrite (BaFe 12 O 19 ) and (JCPDS no: ) [13], for barium cobalt iron oxide (BaCo 2 Fe 16 O 27 ). It can be seen from the Fig. 3a that the initial BaM compound is of a pure single phase and agrees well with the standard , including similar relative intensity profiles for the first three strong peaks. The peaks were indexed as a primitive hexagonal cell with space group, P63/mmc (194) with the refined lattice parameter values of a = b = Å and c = Å. These values agree well with the published literature data for BaM [14 16]. From the point of view of phase evolution in this system the trend can be divided into three distinct regimes. When Co 3 O 4 doping is made up to 3% by weight (Fig. 3b), there is no change in the crystal structure and matches with the standard for BaFe 12 O 19 (M-type). However, when the doping exceeds this value, for example in Fig. 3c of 10% there appears to be relatively no structural change up to 5% doping of Co 3 O 4. However, beyond the limit there is an evolution of a distinct phase change, in that the Fig. 3 X-ray Diffraction patterns of sample co-precipitation method compound sintered at 1200 C/2 h, Standard JCPDS File ( ) and ( ) (a) the pure BaM (b) BaM compound with 3% Co doping, (c) BaM compound with 10% Co doping, (d) BaM compound with 30% Co doping parent phase is now converting to this new phase, BaCo 2 Fe 16 O 27, (W-type) as shown in Fig. 3d. It can be seen from Fig. 4, for samples doped from 0 10% from the full width half maxima (FWHM) of the 100% peak (107) for BaM ( ) and the 100% peak (116) in the 20 30% doping compound ( ). Fig. 4 Effects of doping concentration on the BaM 100% peak (107) position from XRD results

5 J Mater Sci: Mater Electron (2007) 18: Table 1 Summary of XRD data and crystallite size Doping Co 3 O 4 (%) 100% peak position (deg) FWHM (deg) JCPDS: JCPDS: Average crystallite size (D) (lm) There is a progressive peak shift to higher angles indicating slight changes in the lattice parameters. Also, the crystallite size, D as determined from FWHM has been summarised in Table 1. The crystallite size remained unchanged for all doping levels and the average crystallite size was found to be approximately 5 lm. The phase conversion from M to W-type is much more apparent as the level of doping is further increased. Here, this new phase now replaces the old parent phase and is identified as a compound based on a stoichiometric composition corresponding to JPDCS file no; Clearly, there is a limit as to how much Co ions can be incorporated in the parent phase before the parent phase dissociates, reacts with the excess Co 3 O 4 and perhaps simultaneously converts to this new W-type phase. Figure 5a d show representative SEM micrographs of the phase pure BaM (0%) and 2, 15 and 30% doping of Co 3 O 4 pellets after sintering at 1200 C for 2 h at a magnification of The particles were shown to have the following dimensions; 0% doping between 5 and 17 lm, 2, 15 and 30% doping between 3 and 5 lm. 4 High frequency dielectric properties The dielectric characteristics over frequency of the BaFe 12 O 19 hexaferrite prepared by co-precipitation doped with Co 3 O 4 at the various weight dopant percentages are shown in Figs All the measured data refers to a frequency range of 1 MHz 1 GHz at the ambient temperature. These plots refer to the theoretical assumption that, when an alternating current electric field is applied to a dielectric material it causes loss and a delay to the dielectric response to the electric field; this defines the complex relative permittivity in an ac electric field. The real part of the complex permittivity (e r ) represents the quantity of stored energy in the dielectric material from the ac field. The related plot of the Fig. 5 Representative micrograph of (a) Pure BaM (b) BaM compound with 2% Co doping (c) BaM compound with 15% Co doping and (d) BaM compound with 30% Co doping

6 532 J Mater Sci: Mater Electron (2007) 18: Fig. 6 (a) Effects of doping concentration, sintering temperature and duration on the mean average value of the real part of complex relative. (b) Concentration dependence of the real part of complex relative permittivity for a typical sample within the batch population sintered at 1200 C for 2 h mean average value of e r versus percentage Co 3 O 4 doping by weight, sintering time and temperature is shown in Fig. 6a. The frequency dependency of e r over the range of 1 MHz to 1 GHz was found to be invariant throughout the range of doping regimes used, as can be seen by a typical sample in Fig. 6b. The e r value for the lower temperature sintering regimes of C ranged between 6 and 9 until the final sintering temperature of 1200 C when the undoped BaM samples had an e r value of 15. There was a subsequent reduction in e r value with increased doping up to 5 10% doping. Here the e r value stabilised at 9.1. This would suggest that between 5 and 10% doping of Co 3 O 4 the material is already saturated and the Co ions are no longer able to fill iron sites and the structure has therefore changed. This is further corroborated by the TG/DTA and XRD results. A typical plot of samples sintered at 1200 C is shown in Fig. 6b, displaying an e r value that had a very low standard deviation over frequency in the range of MHz. Figure 6a and b both indicate a slight increase in the mean value of relative permittivity within the 30% doped samples over the 10 20% Fig. 7 (a) Concentration dependence of the mean average value of imaginary part of complex relative permittivity for sintering temperature and duration. (b) Concentration dependence of the imaginary part of complex relative permittivity for a typical sample within the batch population sintered at 1200 C for 2 h doped. However, this is only a very slight increase and falls largely within the measurement error of the test equipment highlighted above. If the measurement was representative of the actual response then the slight increase in permittivity value of the 30% doped samples may be due to the accompanied increase in density that occurs over the 10 30% Co 3 O 4 concentration samples. The imaginary part of the complex permittivity (e r ) represents the loss to the ac electric field. Figure 7a shows a plot of the mean average value of e r over frequency against percentage Co 3 O 4 doping by weight, sintering time and temperature. At the low sintering temperature of 900 C for 4 h, the (e r ) gradually increases with Co doping from a value of 0.04 with no dopant to at 30% Co doping. With increased sintering temperature and duration the value of (e r )is reduced to an average of Upon the final sintering temperature of 1200 C for 2 h, and low doping levels of 0 to 5% the (e r ) value is slightly elevated and peaks at 1% doping. But, by 5% doping the value returned to

7 J Mater Sci: Mater Electron (2007) 18: C. The response shown in these figures are the quotient of (e r ) and e r for isothermally and isochronally treated samples, and as such this loss parameter follows the same trends as the e r as the e r values are largely invariant over the frequency range. It has to be mentioned here that the temperature differential between the dielectric measurements (up to 1200 C) and the TG/DTA measurements (up to 1150 C) corresponds merely to 50 C. It is strongly suggested however that this minor temperature differential is unlikely to have a significant effect on the dielectric or magnetic properties of the material. 5 Conclusions Fig. 8 (a) Effects of doping concentration, sintering temperature and duration on the mean average value of the dielectric loss tangent over time and temperature. (b) Concentration dependence of the dielectric loss tangent for a typical sample within the batch population sintered at 1200 C for 2 h quite a low value of and remained at this value until a 30% doping level where a sharp increase was observed as the (e r ) was found to be A typical plot of samples sintered at 1200 C is shown in Fig. 7b, displaying an invariant response of (e r ) over frequency in the range of MHz. Evident in Fig. 7b between 1 50 MHz is a large peak, where the value of (e r ) begins at 2.94 then reduces to 0.5 at 50 MHz. This phenomenon may be due to a resonance within the material. Observation of the lower frequency response would clarify the phenomena, however, due to test equipment bandwidth limitations an in depth study cannot be performed at this time. The loss tangent, tan d is the ratio of the imaginary part to the real part of the complex relative permittivity. The tangent of the phase angle d of the complex relative permittivity is defined in terms of e r and (e r ). Thus, the Fig. 8a that shows a plot of the mean average value of loss tangent versus percentage Co 3 O 4 doping by weight, sintering time and temperature and Fig. 8b showing a typical plot of samples sintered at The high frequency dielectric characteristics of the barium hexaferrite (BaM) have been recorded. Additionally, dielectric characteristics after having been doped with Co 3 O 4 at 1, 2, 3, 5, 10, 15, 20 and 30% by weight as a function of temperature (isothermal), time (isochronal) and frequency, as well as density, thermal and crystalline characteristics when prepared by co-precipitation and solid state synthesis routes were provided within this paper. The dielectric properties were found to have a very low standard deviation about the mean value over the frequency range of MHz. A considerable difference was observed during the doping of Co ions by percentage weight in the Fe sites. The initial value of relative permittivity was 15.1, without doping. This value dropped upon doping where between 5 and 10% doping by weight the relative permittivity stabilised at a value of 9 and then only affected by density variations whereupon a slight increase was observed. However, the loss tangent was only affected between 20 and 30% doping levels where it increased from to , possibly also due to increasing density. The phase evolution of the compound from an M-type BaM to a W-type barium cobalt ferrite oxide is significant when higher levels of doping are considered. When designing high density recording media, radio frequency absorbers or surface mount chip devices, it is important to take into consideration the level of multivalent Co in Co 3 O 4 as it has a profound influence on altering the structural makeup, and therefore the electrical characteristics of the final compound. Acknowledgements The authors would like to thank Mr Martin Davies for his assistance in the Materials Preparation and Microscopy Laboratory, School of Engineering, University of Warwick, and for Electron Microscopy and XRD of the dielectric materials. Additionally our thanks are due to Mr David Hammond for the use of the TG/DTA equipment in the

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